Based on the rulebook context, credits are described as a resource that can be used as a "wild" for titanium, ice, and nanocarbon. This means that credits can substitute for these resources when paying costs, but the rulebook does not specify that you can directly trade resources for credits (money).

There is, however, an action at the Trade Hub Alliance Base:

• "Spend any 2 resources to gain 2 credits."
  (Core Rulebook, Page 10)

This means you can trade any combination of resources (energy, titanium, nanocarbon, ice) for credits at the Trade Hub.

Summary

• At the Trade Hub Alliance Base, you may spend any 2 resources (energy, titanium, nanocarbon, ice) to gain 2 credits.

Source: Core Rulebook, Page 10.

Sources:

• Core Rulebook (pp. 10, 16, 7)
• Appendix (pp. 19, 20, 21, 6, 7)